Heretofore, a lead storage battery has been extensively used as a secondary battery. This is due for example to the fact that a lead storage battery can be manufactured at low cost and is comparatively easy-to-use and so on. The lead storage battery, however, uses lead which is a material hazardous to a human body and further has a low electric power density, thus resulting in a limitation in application. Under the situation like this, recently, a lithium-ion battery which has high electric power density as an electricity storage medium and can be reduced in size is coming into wide use. FIG. 8 represents a structure of a laminate-type lithium-ion battery, as a one exemplary configuration of the lithium-ion battery. A lithium-cell 1, acting as the laminate-type lithium-ion battery, has a structure such that a separator 4 is inserted for insulation between a positive-electrode material 2 such as lithium cobaltate (LiCoO2), lithium manganate (LiMnO2) or the like and a negative-electrode material 3 such as graphite (carbon) or the like. Then, some of thus assembled units are stacked into a laminate structure 5, and thereafter the laminate structure 5, together with an electrolyte, is sealed with aluminum laminates 6 from upper and lower sides thereof. The positive-electrode material 2 and the negative-electrode material 3 are formed with a positive electrode 2a and a negative electrode 3a, respectively, and both the electrodes protrude outward from the bonded portions of the aluminum laminates 6. In the meantime, a protruding form, figure and material of the electrodes, an entire size of the laminate-type battery or the like are not specifically limited, permitting various types thereof.
Despite high electric power density as an electricity storage medium, the lithium-cell 1 with the structure like this uses a material which generates a flammable gas at high temperature and hence involves the risk of inducing smoking and kindling if abnormal state such as overcharge or the like occurs in the battery. Flammable internal gases generated inside the lithium-cells 1 include evaporating gases (diethyl carbonate and ethylene carbonate gases) generated from the electrolyte due to, e.g., overvoltage and CH4, C2H4, C2H6, etc. which are generated from the electrolyte due to thermo-runaway. Consequently, when the lithium-cells 1 are at high temperature, these gases are generated inside the battery to induce expansions of the aluminum laminates 6. If such condition persists, the aluminum laminates 6 are likely to explode, posing a risk. Then, when the aluminum laminates 6 have been broken, the electrolyte outflows, and when at high temperature, it evaporates to generate visible smoke. Further, if the abnormal state such as overcharging or the like continues, the separator 4 inside the lithium cells 1 melts to cause internal short-circuiting between the positive and negative electrodes 2a, 3a, thus posing risk of giving rise to kindling at once. Accordingly, in order to avert the above-mentioned situations, the lithium-cell 1 has a safety valve 7 provided at an opposite end of a position where the positive and negative electrodes 2a, 3a are provided. Specifically, when internal pressure of the lithium-cells 1 has become equal to or higher than a given pressure, a minimal-length path 7b leading to a valve opening 7a provided in a thermally bonded portion of the aluminum laminates 6 is opened to discharge the internal gases from the valve opening 7a to the outside of the lithium-cells 1.
FIG. 9 represents a battery stack 8 built up by stacking the lithium-cells 1 as a battery cell. The battery stack 8 is housed in a resinous or metallic case (not shown) designed in view of necessary electric energy as a battery pack. Accordingly, if the abnormal state such as overcharge or the like has occurred in the battery stack 8, the internal gases evaporated from the lithium-cells 1 are discharged and as a result fill an inside of the case of the battery pack. Then, a pressure rise inside the case causes outward leakage of the internal gases. At the same time, if the case is formed with an air-tight structure to prevent the gas leakage, internal pressure of the case rises, resulting in a risk for the case to be broken in the worst-case scenario.
In view of the above-mentioned, one scheme has ever been proposed (for example in patent document 1) in which an explosion-proof valve is provided in a case itself of the lithium-ion battery and when pressure inside the battery has risen to a given pressure or above, internal gases inside the battery are discharged to the outside of the battery through the explosion-proof valve to perform gas venting, thus preventing the gases from exploding.
Patent document 1: Japanese unexamined patent application publication No. 11-283599